Linear encoders for precise distance measurement are required in many fields of application, in which the position of an element movable along a linear path, for example, the position of a machine component on a linear axis such as drives or pivot arms, is to be determined. The positions detected in this case can be used as position values for measuring purposes, or also for positioning components by way of a drive having a position control loop. Such linear position encoders are accordingly used in devices such as coordinate measuring machines (CMM), geodetic devices, robot arms, or hydraulic actuators. A linear encoder has for this purpose a mass embodiment and a read head for the scanning thereof, which are movable in relation to one another, as well as a control and analysis unit for regulating measurement operations and for assigning a position value, which is recorded by the read head, to a scanning signal. In this case, depending on the requirements and structural options, either the read head is stationary and the mass embodiment is movable, for example, in that the mass embodiment is connected to a movable object, the linear movement of which is to be detected, or the mass embodiment is fixedly positioned and the read head is moved in relation thereto, for example, in that a measuring carriage, which is provided with a read head, of a coordinate measuring machine is moved over a scale carrier attached to a measuring table.
The scanning of the mass embodiment is performed in this case in a contactless manner, generally based on optical, inductive, magnetic, or capacitive physical principles. For example, the read head can have illuminating means, which irradiate a mass embodiment having light-reflective or light-scattering (reflected light scanning) or light-transmitting (transmitted light scanning) markings with light. The light is incident therefrom on a light-sensitive pickup of a sensor, for example, on a photocell or a CCD array. If the mass embodiment has light-reflective markings, the sensor is attached on the same side of the read head as the illuminating means. In the other case, the read head laterally encloses the mass embodiment and sensor and illuminating means are arranged approximately opposite to one another in/at/on the read head. The markings are used as code elements for coding the relative position of the read head in relation to the mass embodiment. The coding can be implemented as an incremental code, for example, by alternating similar light/dark transitions, moiré patterns, or, for measurement methods other than optical measurement methods, as an electrical or magnetic poles, or as an absolute code, for example, by a number of defined different patterns. Combinations of an incremental and absolute partial code are also known, for example, for determining a coarse position and fine position in each case. Possible implementations of an optical position code, for example, are found in patent application EP 12175130.9, for example.
In principle, various materials are suitable as the material for a mass embodiment. Since the code elements are to stand for defined position values, they must actually be located at the location in relation to the read head representing the respective position value. Currently, such mass embodiments are frequently manufactured from steel strips, plastics, glass, or ceramic. The marking is applied with high precision on the mass embodiment for precise localization, which places correspondingly high demands on the production method of the mass embodiment with respect to the manufacturing tolerances, but also, depending on the desired degree of precision, on the calibration procedure required later, which assigns a position value to each code value, which is then typically stored in a storage unit of the linear encoder. It is apparent that after a localization which has been performed once, for example, by code calibration, the code position should remain constant in relation to the read head and should not change. However, in the case of the materials presently typically used for mass embodiments, environmental influences, such as temperature and moisture variations, and also aging processes of the mass embodiment material, result in changes which cause expansion or shrinking of the mass embodiment and therefore stretching or compression of the distances of the code elements to one another. Due to such influences, it can then occur that the position value assigned to a code element by the storage unit no longer corresponds to the actual relative location of read head and measuring rod. In published application DE 19608978A1, using a glass ceramic as the material for the mass embodiment of a light electrical position measuring unit was suggested to avoid such influences.
If tensions arise, as a result of aging or environmental influences, between the material of the mass embodiment and the material of the object accommodating the mass embodiment, for example, a measuring table, the mass embodiment as a whole can thus shift in relation to the read head, whereby the zero point location changes. To counteract such problems, a combination of materials having different coefficients of thermal expansion such that temperature influences cancel out as a whole was proposed (see, for example, DE 19726173 A1). Many of these examples, and also the attempts to compensate for errors caused in this manner mathematically, cf., for example, WO 9935468 A1, require a temperature determination for the correct compensation, however, which presumes a preceding temperature calibration at the producer.
EP 1195880 A1 discloses a method for increasing the positioning precision of a positioning element, which is arranged so it is linearly movable in relation to a read head, of a linear motor, which has a mass embodiment. The mass embodiment has an incremental position code made of magnetic code elements. The read head has a control and analysis unit and at least two sensors spaced apart from one another, which are implemented in this example as Hall sensors and are used for calibrating the positioning element. The distance of the sensors is adapted in this case to the distance which two magnetic code elements on the mass embodiment are to have to one another and is equal thereto in the ideal case. In a calibration run, the positioning element is moved in relation to the read head and code elements are detected by both sensors and output as sensor signals. On the basis of the sensor signals and stored information, target position values are ascertained by the control and analysis unit. In each case a target distance is formed from the difference of that of the two target position values, which result in the case of two positions of the position element, which successively result in an equal sensor signal value at both sensors (within one sine period). Since the actual distance of two such positions corresponds in absolute value to the distance of the two sensors from one another, a target position error is determined from the difference of the target distance and the actual distance. Proceeding from a zero position, this is carried out successively for all successive positions of the positioning element, wherein the preceding difference value is added to the difference value ascertained at the present position in each case.
The method described in EP 1195880 A1 has the disadvantage that the distance of the sensors used as the standard is approximately as great as the marking distances of the incremental position code. This requires a large number of successive steps during the calibration, whereby a large number of error addition steps occur, which cause an uncertainty which increases with the number. In addition, this uncertainty can be amplified by small disturbances in the sensor signal detection, which must be counteracted by error weighting. The apparently required position precision is approximately >1·10−4, which is inadequate for high-precision position or distance measurements, as are required, for example, in applications such as quality testing, meteorological measurements, geodetic surveying, etc. Further disadvantages are that because of the method, an uncalibrated range remains in existence at the zero position of the device, and the distance of the sensors and the distance of the code elements on the mass embodiment must be adapted to one another. In addition, environmental influences or aging phenomena are not taken into consideration in EP 1195880 A1. Nevertheless, these could cause a change of the distance of the sensors to one another, whereby the error value ascertainment can be corrupted very disadvantageously.
The problem of the present invention is therefore to provide an improved, more reliable linear encoder, and an improved calibration method for such a linear encoder.
This problem is solved, or these solutions are refined, according to the invention by the features of the independent claims and/or by features of the dependent claims.